I talked with some folks about this yesterday. This is a quick Blog to summarize what i found. I'm not sure that we have "uniformity" on the issue - but we'd like to get there!
So we are stuck with dealing with Appendix "D" concerning embed items including holdowns. It used to be a few years ago, that we would install a HD, size a plate for "idealized" shear cone, and be done with things... Now its a little different...
In App "D". the are various failure modes - one of which is described in D5.2:
Concrete breakout of anchor in tension. At the end of this section, section D5.2.9 indicates that "... where anchor reinforcement is developed in accordance with Chapter 12..." and goes on to say that you can design "break-out cone" to be "restrained" with rebar per chapter 12. Chapter 12 deals with development and lap splices of rebar.
You'll recall that Rick Fine was with us a while back and talked about a studrail solution and a "rebar chair" solution to the HD question. Studrails can be expensive, have lead times, and can be unwieldy to consider - especially at edge conditions. (incidentally - the stud rail option would be making use of the provisions of Ch11 for shear - not exactly what App "D" had in mind)
Rick also looked at the "rebar chair" option. The problem with this is that the chair legs need to be developed per Ch12. This can lead to some complications - one of which was Rick suggesting to use 14" thk decks (apparently mostly on account of chair dimensions... This does not seem like the right approach to me).
We have recently used closed stirrups with longitudinal rebar as shown here. It seems to me that this approach will work, but there are some difficulties in placing the "large" configuration where tendons are concentrated, and where there may be obstacles that "interfere" with the spacing of stirrups etc. - thereby compromising the whole idea... One idea to improve on this concept may be to have the stirrups be two part elements, with a closure or "hat" bar on top to facilitate installation and other entities that may encroach on the "arrangement" . I like this config at edge conditions though, and so may end up with various embedment "reinforcement" details depending on location and size of holdown force.
Steve has used the "bent bar" approach shown here which i really like: it is simple, easy to install and should work well for our purposes to restrain the "concrete cone". It uses the provisions on Ch11 for shear friction reinforcement, which i think is an appropriate approach when applied to HD conditions we see all the time. This solution is well suited to typical slab conditions "in the field" og the slab... at slab edges, we would still need to scratch our heads. For slab edges where there are walls below, we can use the "lap length" approach as noted below.
At Cantilevered edge conditions, we could use a detail similar to this one. We should probably try to limit the holdown sizes on cantilevers, and try to terminate them in walls below if necessary.
I feel this is something we should try to standardize on, so that we have a uniform approach to holdown reinforcement across projects and across clients. If you have thoughts on this issue please weigh in! Also weigh in the Ch11 vs Ch12 thing too...
